ES372210A1 - Control circuits - Google Patents

Abstract

A pulse generator for signalling each sign change of an A.C. voltage or the zero crossover points has a transistor 10 connected so that when turned on by the A.C. voltage it reduces the bias of a transistor 20 towards cut off, the transistor 20 tending to be cut off during both half cycles of the A.C. voltage applied at 14, 34 and the transistor 20 is biased towards conduction from a D.C. source 24 so that its tendencies to he cut off are overridden to produce a signal output 44 when the A.C. voltage passes through zero. The circuit operates so that for positive half cycles of A.C. the transistors 10 and 20 are on and off and for negative half cycles transistors 10 and 20 are off but during the zero crossover of the A.C. voltage transistor 20 is caused to be momentarily conductive due to the D.C. bias connection via resistor 26. A negative output is obtained at 44. Alternatively a positive going output may be obtained from the emitter (Fig. 2, not shown). The circuit may be used to phase control the firing point of a triac 50 (Fig. 3) used to supply power to a universal motor 60. At the zero crossovers of the A.C. transistor 20 conducts and discharges capacitor 40 which then recharges via resistor 38 to provide a triggering pulse via diac 54 to turn on triac 50 a predetermined time after the zero crossing point. This predetermined time and the power supplied to the load is controlled by the setting of resistor 38. A feedback circuit for speed regulation comprises transistor 30 and diode 68 which sense the current through the load 60 and compensates for any changes by altering the RC time constant circuit to effect the point at which the triac turns on during subsequent half cycles. Resistor 70 and capacitor 72 form a commutating circuit. In a modified circuit for an induction motor load (Fig. 4, not shown), the feedback circuit 30, 68 is omitted and the control circuit transistors 10, 20 are connected after the load so that the discharge of the capacitor 40 is linked to the zero crossovers of the inductive load current which may log the supply line voltage 14, 34. In an alternative control circuit (Fig. 5) for a heater load 60 pulses are applied to the gate of the triac 50 in synchronism with the zero crossovers. The number of half cycles for which the triac 50 conducts is controlled by a transistor 90 which is controlled to be off and saturated under control of an adjustable pulse generator 100.